Design Of Primary-side Feedback Flyback LED Driver Based On Drain-detection Of Power MOSFET

As the fourth generation light source,LED has the advantages of energy saving and environmental protection,high brightness,long life,small volume,lower weight,constant light,no stroboscopic and fast response.However,LED driver is more complex than other light sources,poor performance of which will lost the advantages of LED.Therefore,the study of high reliability,high efficiency,high power factor,small size,low-cost LED drive power has important practical significance.Based on primary-side feedback topology,whose volume becomes bigger and cost higher because of using the auxiliary winding scheme,the accuracy error increases due to leakage inductance of auxiliary winding,this thesis proposed a primary-side feedback and flyback LED constant current driver based on the drain of power MOSFET detection technology without auxiliary winding.The LED driver works in discontinuous conduction mode(DCM)and critical conduction mode(CRM),get the secondary side diode conducting time information by detecting the oscillation signal which is generated by the primary inductor and the power MOSFET's parasitic capacitance at the drain of power MOSFET.In the chip,valley conduction circuit is added to detect the valley voltage of the drain voltage of the power MOSFET and generate the chip set signal to turn on the power MOSFET.At the same time,used the valley-hopping technology in the light load conditions to prevent the circuit from working under very high operating frequency,resulting in low electromagnetic interference and efficiency problems,thereby,reducing switching loss and improving system efficiency.To achieve high power factor,this thesis designed the circuit that the product of the duty cycle and on-time is constant,without the multiplier and other complex circuits,realizing a single-stage active power factor correction(APFC).Combined with the primary peak current information,achieved constant current output and high precision constant current based on the output current estimation method finally.In order to verify the correctness of the proposed scheme,this thesis analyzes the application system and the chip definition and design for the output of 350 mA,10W-15 W system specifications.In the Cadence Specter simulation environment,the important modules in LED driver chip are designed in transistor-level based on the CSMC 0.25 um BCD process,simulated its function.The simulation results show that the accuracy of the output current is within 2.57%,the power factor PF is above 0.99,and the system efficiency is over 90% in range of universal-input ac voltage from 85 V to 264 V at 50 Hz.